IGD have recently been approached by a client who operates a laboratory complex with Liquid Nitrogen storage. The Liquid Nitrogen is decanted into dewars for use by staff in a number of different areas within a building.
The client perceived that the currently installed gas detection system, from a competitor, was having issues with false alarms as they were beginning to see repeated triggering of alarms during what they saw as normal activities. Their expectation was that the gas detection system was there to report exceptional circumstances of gas leak and should not be activating during normal working practices. The client asked IGD for a solution for oxygen detection system.
The client was concerned that the detectors were placed incorrectly for the site operations and alarms were too close to “normal” levels for their activities.
IGD visited site and undertook a Gas Hazard overview of the site and activities. At the same time IGD reviewed sensor placement (undertaken by consultants at the design stage). Oxygen detectors showed a range of different readings from 19.8-20.9%v/v Oxygen (atmospheric Oxygen is 20.9%v/v Oxygen); the site alarms were all set at 19.5%v/v for room evacuation.
Problem with Lead Based Oxygen Sensors
What was found is typical of older technology “lead based” sensor systems. With this type of ’legacy’ sensor as the Oxygen detectors age so their readings and the reliability of their readings deteriorate.
Lead based Oxygen detector technologies consume their working electrode over the life of the sensor. As more of the surface of the working electrode is reacted from lead to lead oxide so the ability for the cell to produce sufficient signal output is reduced. This is typically exhibited by the detectors gas readings drifting low and towards the alarm point.
The initial effect is to reduce the difference between normal and alarm point so small gas releases will cause unwarranted alarms.